No zero possible in .fill in the blank. ?

Hi,I've made soms questions where students have to fill in the blank. They should fill in numbers only in the entry field because the questions are about calculating stuff. I gave students the freedom to use ',' or '.'and some decimals. This works fine.There is only one problem: when the answer should be zero (0), the control option (if question x is right or wrong, this or that happens) cannot handle the answer. When the student leaves the blank empty, it's just a wrong answer. But when he or she fills in zero, nothing happens! I could use 0,0 or 0.0 but nobody will think about those answers :-)Does anyone has a solution for this problem?Thanks in advance!Greetings, Ellen (Holland)